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        //for 循环的执行过程
        console.log('staat');
        for (var i = 0; i <5 ; i++) {
            console.log("hello js");
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        console.log('end');
        /* 
        执行顺序:
          1.初始化条件:var i=0;(只执行一次)
          2.判断条件: i < 5
            2.1 true ————>逻辑体————>变化条件————>判断条件
            2.2false
               跳出整个代码块
        */
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